The author directly states that this idea is the primary reason for all he has written so far. Christianity is earth's only religion that is led by a spiritual High Priest sitting at the right hand of the throne of God in heaven. Within the material the author has written are two major points:
First, the qualifications of this towering Figure, who holds such an important office, make Him indispensable to the salvation of all God's sanctified ones. Indispensable? Absolutely! Jesus tells us Himself in John 15:5, “Without Me you can do nothing” in terms of producing fruit that glorifies God. He has much to offer. The epistle to the Hebrews identifies these qualities.
The second major reason is not named here. Some may consider it unimportant in comparison to the first. However, God, who knows precisely where His creation is headed and who sovereignly controls its direction and speed of advancement, never intended the Old Covenant to last forever.
Remember, God Himself publicly introduced the New Covenant six centuries before the writing of the book of Hebrews (Jeremiah 31:31-34). Its introduction within the flow of the history of the church and the world began to force key cultural changes to take place within Judea especially, but also in majority Gentile areas of the Middle East. Many Jews were being converted. Within the church itself, both the leadership and membership were asking many questions about what they needed to do to adjust to this new way of life. Those converts required direction from on high to secure them in living by faith in Jesus Christ.
The transition from Judaism to Christianity following Christ's crucifixion and resurrection and the church's receipt of the Holy Spirit—all in the early AD 30s—needed purposeful instruction from heaven to confirm to the church the direction that Christ wanted the daily, spiritual operations of Christianity to proceed. Just as the book of Leviticus contains detailed instruction for daily functions under the Old Covenant, so similar education was necessary under the New Covenant because of what God was working in the church—and is still working today.
The epistle to the Hebrews contains such instruction, enabling those who have entered the New Covenant with God to make the necessary adjustments to maintain their lives by faith and grow spiritually. In this way, they can glorify God by maintaining their relationship with Christ while preparing for the Kingdom of God.
John W. Ritenbaugh
Why Hebrews Was Written (Part One)